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HERA GETS GOING WITH ELECTRONS

August 11, 1998

The Hadron Electron Ring Accelerator (HERA) at the DESY
laboratory in Hamburg is about to begin operations for 1998
with a return to electrons. Since 1993, the machine has
been producing head-on collisions between high-energy protons
and positrons - the antimatter equivalent of electrons.
Now it is ready to get going with electrons again.


In the past, electron beams would not coast round the 6.3 km
ring of the HERA collider for as long as positron beams. This was
because electrons, with negative electric charge, would
attract residual positive ions within the beam pipe leading to
beam loss. On the other hand, positrons, which have positive charge,
would repel the positive ions and could make many more turns round the
machine.

To improve the lifetime of stored electron beams, new vacuum pumps
have been installed around the electron beam pipe removing more of
the rogue positive ions. Physicists working at HERA now look forward
to collecting as much data on electron-proton collisions as they have
already amassed on positron-proton collisions.

"Having both positron and electron beams will open up a
wealth of physics opportunities at HERA," says Professor Roger Cashmore,
Chairman of Physics at Oxford University and spokesman for
UK scientists in the ZEUS collaboration.


Physicists working at HERA have been probing the complex tangle of
particles called quarks and gluons inside the proton - a basic building
block of all atomic nuclei. At a fundamental level, electrons and
anti-electrons should interact in the same way with the quarks and gluons,
so in many ways the changeover to electron-proton collisions should not
make a significant difference.

However, one reaction in particular can occur more readily with electrons.
This is when the electron interacts with a quark and changes into the
elusive particle known as the electron-neutrino. These interactions occur
through the weak force - the same force that underlies the initial
step in the chain of nuclear reactions that make the Sun and other
stars shine. At the high energies of the electron-proton collisions
at HERA, such weak interactions occur as often as electromagnetic
interactions - a result of the underlying connection between these
two fundamental processes, known as "electroweak unification".

"This is the physics HERA was designed for," says Professor Cashmore.

UK physicists play an important role in the two major experiments
at HERA, called H1 and ZEUS. Professor John Dainton (Liverpool
University) is spokesman for the H1 experiment, and Professor
Brian Foster (Bristol University), will become the spokesman for
ZEUS in January 1999.

For further information on work at HERA, please contact:

Professor Roger Cashmore, Oxford University
Tel: 01865 272276
Fax: 01865 272382
email: r.cashmore1@physics.ox.ac.uk

Dr Valerie Noyes, Oxford University
Tel: 01865 273356
Fax: 01865 273418
email: v.noyes1@physics.ox.ac.uk

DESY Press Office
Tel: 00 49 40 8998 3613
Fax: 00 49 40 8998 4307
email: desypr@desy.de


For general information on particle physics:

Dr Christine Sutton, Oxford University
Tel: 01865-273322/273353 or 01235-850091
Fax: 01865-273418
email: c.sutton1@physics.ox.ac.uk

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